1. (a) For purposes of this Rule, the following definitions shall apply:

   1. (1) "1Q10" means the minimum average flow for a period of one day that has an average recurrence of once in ten years;
   2. (2) "7Q10" means the minimum average flow for a period of seven consecutive days that has an average recurrence of once in ten years;
   3. (3) "30Q2" means the minimum average flow for a period of 30 consecutive days that has an average recurrence of once in two years;
   4. (4) "Mean annual flow" means the same as "annual mean flow" as defined in 40 CFR 125.83, which is incorporated by reference including subsequent amendments and editions and available free of charge at: https://www.ecfr.gov/current/title-40/chapter-I/subchapter-D/part-125/subpart-I/section-125.83;
   5. (5) "Non-erosive velocity" means the same as defined in 15A NCAC 02H .1002 (26); and
   6. (6) The "Rational Method" estimates peak flow for a storm of interest as a function of a composite runoff coefficient, rainfall intensity for the storm of interest, and drainage area.
2. (b) Water quality based effluent limitations shall be developed by the Director to allow appropriate frequency and duration of deviations from water quality standards so that the designated uses of receiving streams and downstream waters are protected. There are water quality standards for a number of categories of pollutants and to protect a range of water uses. For this reason, the appropriate frequency and duration of deviations from water quality standards shall not be the same for all pollutants. A flow design criterion shall be used in the development of water quality based effluent limitations as a simplified means of estimating the acceptable frequency and duration of deviations. More complex modeling techniques may be used to set effluent limitations based on frequency and duration criteria published by the U.S. Environmental Protection Agency and incorporated by reference, including subsequent amendments and editions. Frequency and duration criteria published by the U.S. Environmental Protection Agency is available free of charge at: http://water.epa.gov/scitech/swguidance/standards/criteria/current/index.cfm. Where the Director determines that the applicant selected model will not protect the designated uses of the receiving streams and downstream waters, then the Director shall notify the applicant in writing the reasons why the selected model is not suitable.

3. (c) Water quality based effluent limitations shall be developed using the following flow design criteria:

   1. (1) Except for toxic substances and aesthetics, all water quality standards shall be protected using the 7Q10 flow. Other governing flow strategies, such as varying discharges with the receiving stream's or downstream water's ability to assimilate wastes, may be designated by the Director on a case‑by‑case basis if the discharger or permit applicant provides evidence that establishes that the alternative flow strategies will give equal or better protection of water quality standards such that deviations from the standard would be expected at the same or lesser frequency than provided by using the 7Q10 flow.

   2. (2) Toxic substances shall be protected as follows:

      1. (A) Toxic substance standards to protect aquatic life from chronic toxicity shall be protected using the 7Q10 flow;
      2. (B) Toxic substance standards to protect aquatic life from acute toxicity shall be protected using the 1Q10 flow;
      3. (C) Toxic substance standards to protect human health through the consumption of water, fish, and shellfish from noncarcinogens shall be protected using the 7Q10 flow; and
      4. (D) Toxic substance standards to protect human health through the consumption of water, fish, and shellfish from carcinogens shall be protected using the mean annual flow, unless site specific fish contamination concerns necessitate the use of an alternative design flow.
   3. (3) Aesthetic quality shall be protected using the 30Q2 flow.
4. (d) If the stream flow is regulated, a minimum daily low flow may be used as a substitute for the 7Q10 flow, except in cases where there are acute toxicity concerns for aquatic life. For streams where there are acute toxicity concerns, an alternative low flow, such as the instantaneous minimum release, shall be used if the Director determines, on a case‑by‑case basis, that the designated uses of receiving streams and downstream waters are protected.
5. (e) Flow design criteria shall be used to develop water quality based effluent limitations and in the design of wastewater treatment facilities. Deviations from a specific water quality standard resulting from discharges that are demonstrated to be in compliance with water quality based effluent limitations for that water quality standard shall not be a violation pursuant to G.S. 143-215.6A when the actual stream flow is less than the design flow.

6. (f) If the 7Q10 flow of the receiving stream is estimated to be zero and the 30Q2 flow of the receiving stream is estimated to be greater than zero, then water quality based effluent limitations shall be assigned as follows:

   1. (1) New or expanded discharges of oxygen consuming waste shall be set at BOD₅= 5 mg/l, NH₃‑N = 2 mg/l and DO = 6 mg/l, unless it is determined by the Director through modeling or other analysis that these limitations will not protect water quality standards. More stringent limits shall be applied if violations of water quality standards are predicted to occur for a new or expanded discharge with the limits set pursuant to this Rule or if existing limits are determined to be inadequate to protect water quality standards.
   2. (2) Other water quality standards shall be protected by requiring the discharge to meet the water quality standards set forth in this Subchapter, unless the Director determines that alternative limitations protect the designated uses of receiving streams and downstream waters.
   3. (3) Requirements for existing discharges shall be determined on a case‑by‑case basis by the Director using a site-specific analysis for each discharge.

7. (g) If the 7Q10 flow and the 30Q2 flow of the receiving streams are both estimated to be zero, then domestic wastewater discharges of oxygen consuming waste shall be permitted only if all of the following are met:

   1. (1) The proposed permitted flow for the wastewater discharge shall be lesser of:

      1. (A) No more than one-tenth of the flow generated by the one-year, 24-hour storm event based on the drainage area of the receiving stream at the discharge location and calculated using the Rational Method. The Rational Method shall be used to calculate the peak runoff for the one-year, 24-hour precipitation event in cubic feet per second. The peak runoff shall then be divided by 10 and multiplied by 646,272 to convert the result to gallons per day of allowable discharge at the point studied; or
      2. (B) No more than two million gallons per day.

   2. (2) All wastewater discharges shall be directed to a system that utilizes low-energy methodologies prior to discharging to receiving streams at non-erosive velocities, such as:

      1. (A) An infiltration system, which may include engineered materials to achieve higher rates of infiltration. Engineered materials shall have an ASTM gradation of fine to coarse grain sand and shall be angular to maintain structural integrity of the slope;
      2. (B) Constructed free-surface wetland with a hydraulic residence time of at least 14 days; or
      3. (C) Other technologies that meet the standard of practice for NC Licensed Professional Engineers for such devices that provide a physical buffer or hydraulic residence time sufficient to discharge at non-erosive velocities.
   3. (3) If the wastewater discharge to the receiving stream exceeds one cubic foot per second based on the average daily flow of the discharge, then more than one outfall to the receiving stream shall be utilized for the wastewater discharge. The discharge at each outfall shall not exceed one cubic foot per second and the outfalls shall be at least 50 linear feet apart along the receiving streams. Cumulative discharge to the receiving stream shall not exceed the lesser of the requirements in Subparagraph (g)(1) of this Rule.
   4. (4) No wastewater discharges shall be allowed to Class SA, SB, SC, WS-I, WS-II, WS-III, WS-IV, WS-V, ORW or HQW waters.
   5. (5) For wastewater discharges to NSW waters, the Director may require additional modeling by the applicant using a site-specific analysis for each discharge. Additional allocation of flow shall be at the discretion of the Director using a site-specific analysis for each discharge.

   6. (6) In addition to any other effluent limits for any other parameters to ensure the permit does not violate any EPA-approved NC water quality standards, the following effluent limits shall apply:

      1. (A) Biological oxygen demand (BOD5) shall not exceed 5.0 mg/l monthly average;
      2. (B) NH3, 0.5 mg/l monthly average, 1.0 mg/l daily maximum;
      3. (C) Total nitrogen shall not exceed 4.0 mg/l monthly average;
      4. (D) Total phosphorus, 1.0 mg/l monthly average, 2.0 mg/l daily maximum;
      5. (E) Fecal coliforms, 14 colonies/100ml or less;
      6. (F) Dissolved oxygen, 7.0 mg/l or greater;
      7. (G) Total suspended solids, 5.0 mg/l monthly average, 8mg/l daily maximum; and

      8. (H) Nitrate, 1.0 mg/l monthly average, 2.0 mg/l daily maximum.

         The Director may impose different effluent limits than those set forth in Parts (g)(6)(A) through (H) of this Rule to ensure that the permit does not violate any EPA-approved NC water quality standards using a site-specific analysis for each discharge.

   7. (7) The applicant shall demonstrate:

      1. (A) The proposed discharge meets the requirements in Subparagraphs (g)(1), (2), (3), and (4) of this Rule;
      2. (B) The proposed discharge is a domestic wastewater discharge as defined in Rule .0202 of this Section;
      3. (C) When the receiving stream has naturally occurring low dissolved oxygen levels, the proposed discharge complies with G.S. 143-215.1(c7); and
      4. (D) When the receiving stream does not have naturally occurring low dissolved oxygen levels, the proposed discharge does not reduce the dissolved oxygen levels of the receiving stream more than 0.1 mg/l below the approved modeled in-stream dissolved oxygen level for the receiving stream at total permitted capacity for all discharges to such receiving stream. The applicant shall use a model utilized elsewhere in USEPA Region 4, such as the Streeter-Phelps model used in the State of Alabama, and the selected model shall be approved by the Director as suitable for the particular discharge and receiving stream.
   8. (8) If an applicant requests less stringent effluent limits than those set forth in Subparagraph (g)(6) of this Rule, then the applicant shall conduct more complex modeling. The applicant shall use a model accepted elsewhere in USEPA Region 4 that is approved by the Director as suitable for the particular discharge and receiving stream. The modeling shall demonstrate the requirement in Part (g)(7)(C) or (g)(7)(D) of this Rule, whichever is applicable, is met, and all EPA-approved NC water quality standards are protected.

   9. (9) Applicants shall provide either:

      1. (A) Mapping data from USGS; or
      2. (B) Mapping data prepared by an engineer of record licensed in the State utilizing either USGS mapping data or other maps approved for use by the Director.
   10. (10) Where the Director determines that the applicant selected model under Subparagraphs (g)(5) or (g)(8) of this Rule or Part (g)(7)(D) of this Rule is not suitable for the particular discharge or receiving stream and downstream waters, then the Director shall notify the applicant in writing the reasons why the selected model is not suitable.
8. (h) Receiving water flow statistics shall be estimated through consultation with the U.S. Geological Survey. Estimates for any given location may be based on actual flow data, modeling analyses, or other methods determined to be appropriate by the Director.

History Note: Authority G.S. 143-214.1; 143-215.1(c7); 143-215.3(a)(1); SL 2024-44 s. 5.1; SL 2025-94 s. 23;
Eff. February 1, 1976;
Amended Eff. January 1, 2015; February 1, 1993; October 1, 1989; August 1, 1985; January 1, 1985;
Readopted Eff. November 1, 2019;
Amended Eff. May 1, 2026.